1. Field of the Invention
The invention relates to the field of telecommunications and more particularly to the field of information transfer using packet data switching.
2. Related Art
Circuit switched data is useful for transactions where a relatively large amount of data is transmitted quickly by a relatively small number of users, such as in fax transmissions. The IS-136 standard supports analog and digital circuit switched data. Digital circuit switched data is supported using time division multiple access (TDMA) techniques on digital traffic channels. Analog circuit switched calls use analog voice channels and are essentially transparent to the network.
In contrast, packet switching of data is useful in situations where relatively small amounts of data are transmitted by a relatively large number of users, such as in Internet E-Mail transmissions. The IS-136 standard does not currently support integrated packet switched data transmission.
One approach that has been used as an overlay to IS-136 to support packet data is called Cellular Digital Packet Data (CDPD). CDPD specifies packet data operation that use free analog voice channels to transmit packet data. Efforts are now underway to develop an IS-136 standard to support a TDMA air interface that is compatible with CDPD and will integrate packet and circuit switched voice/data with respect to call delivery, mobility management and authentication.
The use of packet control channels (PCCH) and packet traffic channels (PTCH) has been proposed as one way to implement a packet switched system. In a PCCH/PTCH system, call setup would take place on a PCCH, while packet delivery would take place on a traffic channel (PTCH) assigned from the PCCH. The PCCH/PTCH approach has the disadvantage of idle channel capacity. As previously noted, packet switched data tends to be used when a large number of users have relatively small amounts of data to transmit. The occurrence of bursts of data is characteristic of such situations. Trunking efficiency using PCCH/PTCH is not good for low to moderate offered traffic loads, because capacity is wasted on the PCCH which is only used for call set up signalling.
An alternative approach is dedicated PCCH. Using dedicated PCCH, both control and data traffic occurs on the same channel, thereby avoiding the wasted traffic capacity associated with PCCH/PTCH. There are multiple PCCHs in a cell. A mobile unit or mobile finds its PCCH using a static hashing algorithm based on mobile station identification (MSID). Since the hashing algorithm is static, it is possible for a PCCH to become overloaded. For example, under high load conditions and even under some low load conditions, it is possible for the static hashing algorithm executed in the mobiles to hash too many mobiles to the same PCCH. Overloading can occur, for example, when the hashing algorithm, which is executed in each mobile seeking to communicate, observes a large number of mobiles with the same or similar digits in certain portions of the MSID used by the hashing algorithm. In the overload circumstance, the mobile receives a message from the switch on the selected PCCH to go to another PCCH. A disadvantage with this approach lies in the fact that the message to select another channel must be transmitted on the overloaded PCCH, thus further overloading that channel. It may take several attempts to successfully transmit the necessary information to the mobile from the switch, adding to the overload condition and delaying transmission even longer. Furthermore, the processing for this condition takes place in the switch, which must identify a better channel and notify the mobile to tune to that better channel. As a result, benefits in reduced switch processing associated with locating the scheduling algorithm in the mobiles rather than the switch are lost.